Green tea is drunk as a coarse tea, which is in the form of leaves, or drunk as a fermented tea to feel a deeper flavor. Fermented green tea means that the green tea leaves have been oxidized. It includes fermented teas oxidized by oxidases present in tea leaves and post fermented teas fermented by microorganisms other than the enzymes present in tea leaves. It can be classified into weakly fermented tea, semi-fermented tea, fully fermented tea, etc. according to the degree of fermentation. Fermented green tea is referred to as various names such as green tea, oolong tea, black tea, and pu'er tea depending on the type and degree of fermentation.
The countries with the world's leading technology for producing post fermented tea are China and Japan. The aerobically fermented Chinese teas are produced using fungi as the fermentation strains and mainly cultivated in West Xishuangbanna, Yunnan province. The aerobically fermented Japanese teas include dark tea, which is produced using fungi as the fermentation strains and mainly cultivated in Toyama. The anaerobically fermented Japanese teas include Bancha, which is produced using anaerobic Lactobacillus as the fermentation strain and mainly cultivated in Awa, and black tea, which is produced using anaerobic fungi as the fermentation strains and mainly cultivated in Ishizuchi. Traditional pu'er tea, which originated from the city of Pu'er in China, is produced by deliberately harming green tea leaves when harvesting them, heating and roasting them, applying an appropriate amount of water, and naturally fermenting them using microorganisms in the air. In Korea, fermented teas are produced for consumption intermittently by a cottage industry in Mt. Jiri and Boryeong.
Fermented teas are different from coarse teas in flavor. They may also show a large difference in the type and content of active ingredients depending on the specific fermentation process and the type of microorganism. As such, various compounds can be generated and separated therefrom, and thus various attempts have been made to separate and identify unknown novel compounds using green tea.
Quercetin-based compounds are a type of flavonoids, and most of them exist as glycosides in nature, which include quercitrin, isoquercitrin, quercimeritrin, avicularin, hyperin, reynoutrin, querciturone, and rutin, depending on the type of saccharides.